As the technical and commercial viability of solar photovoltaic technology has increased, the implementation of this technology has boomed. The solar photovoltaic industry is unique in that module manufacturers typically warrant their products for operation over 25 years. Other array components are also often warranted for at least 5 years with extended warranties becoming common practice. While photovoltaic modules and their auxiliary components are reliable, faults can and do occur. This highlights the need for robust and reliable monitoring to ensure that a solar photovoltaic system performs as it should and alert an owner in the event of a fault.
Many monitoring devices and solutions are available on the market today to inform an owner about the performance of their solar photovoltaic system. Of these solutions, there are four main categories of products available to an owner of a solar photovoltaic systems of all sizes; string level, AC output metering, inverter integrated and stand alone units.
For the purpose of this document, a small scale system is defined as a photovoltaic system equal to or smaller than 10 kWp, typically installed on residential buildings, a medium scale photovoltaic system is greater than 10 kWp and equal to or smaller than 1 MWp, typically installed on commercial buildings and a large scale photovoltaic system is greater than 1 MWp and is typically found in free field installations.
String monitoring is typically employed on large scale deployment of solar photovoltaics. This form of monitoring measures the DC power output of input strings on the system. A sample embodiment of this monitoring concept makes use of algorithms and processing techniques to compare similar strings to each other, thereby enabling isolation of any faulty individual string that does not match the whole. This is possible because the DC side of large scale photovoltaic systems have numerous strings available that can be individually monitored and compared. String monitoring requires hardwiring of copper or optical fibre cable from the string monitoring unit to a local computer. The data from the string monitor is then visualised in some form through dedicated software. In recent years, string monitoring solutions have become commonplace in large scale solar photovoltaic systems.
Although commonplace in larger systems where multiple strings are available for comparison, this form of monitoring is ineffective for small to medium scale solar photovoltaic systems. The cause of ineffective comparison monitoring is due to most small scale systems consisting of a single string of solar photovoltaic modules thus not allowing for any comparison. The typical small scale system consists of 2 or fewer strings and comparison between only 2 variables is inconclusive unless a major fault occurs. For medium scale systems, comparison monitoring poses similar problems. Medium scale photovoltaic systems typically have a larger number of strings than small scale systems, but equally as few suitably similar strings are available for comparison particularly since these strings are often unequal, have different installation orientations or different shading impacts.
Another shortcoming of this form of comparison monitoring is that it is not capable of detecting system underperformance where all strings may be uniformly underperforming due to issues such as excessive soiling, whole system shading etc. It is also not capable of detecting inverter faults or underperformance as it is a DC side monitoring solution. This monitoring solution does not provide a means of calculating the expected performance of a photovoltaic system. Instead, it is a means of measuring and comparing outputs and not a measure of overall system performance.
Over the past 10 years, the most common monitoring solution available to small to medium scale solar photovoltaic system owners is the monitoring capability integrated in the systems power conditioning unit, the inverter. This monitoring solution measures the DC output and AC output of the array it is connected to and is capable of detecting system faults such as electrical problems with grounding, overvoltage and grid failure. As with string monitoring, this solution typically requires hardwiring of copper or optical fibre cable from the inverter to a local computer. Recent developments have seen this length of cable replaced with wireless communication either via Bluetooth or a wireless router. However, hardwiring is still the industry norm. Variations of this topology exist for medium scale systems where a dedicated data logger is installed to receive data from multiple inverters, collate the data and then transmit through a single communication port. The system data from the inverter or data logger is then visualised in some form through dedicated software. Some manufacturers also include access to a web portal for a limited view of key performance metrics thus enabling ‘monitoring’ of a system from anywhere in the world. A key short coming of this monitoring solution is that it does not provide a means of calculating the expected performance of a photovoltaic system. It is a means of measuring AC output and detecting major inverter faults and not a measure of overall system performance.
A recent development in the field of monitoring solar photovoltaic systems is that of the stand alone monitoring solution. This solution is not tied to an inverter brand and so is a versatile variation of the inverter integrated monitoring platform. Stand alone units typically relay performance data, operating messages and alerts as displayed by the inverter to a local computer. Some stand alone units newly available on the market take this notion one step further by supplementing the inverter communications with other performance metrics such as performance ratio (PR) and normalised system outputs (kWh/kWp/day). Typically, the stand alone solution will offer both hardwiring and integration into an existing wireless network as communication options. As with both previous monitoring solutions, the system data from the inverter or data logger is then visualised in some form through dedicated software and limited view via a web portal.
Failure in communication in these stand alone units is a common cause of failure. To avoid integration into existing internet networks and to increase reliability, providers of this solution typically offer alternate forms of communications as add-ons such as communications through AC mains circuit or radio frequency transmission. However, this adds significantly to the expense of the monitoring solution.
As with all of the assessed monitoring solutions, a shortcoming of this monitoring solution is that it does not provide a means of calculating the expected performance of a photovoltaic system. It is a means of measuring AC output and detecting major inverter faults and not a measure of overall system performance.
An assessment of currently available monitoring platforms reveals that they lack the depth of analysis required to definitively answer the question “Is this photovoltaic system working as it should?” Monitoring solutions available to the solar photovoltaic system owner today, as described above, have either been designed for large scale systems, and hence are expensive and designed for skilled users, or are too simplistic and limited to provide useful system performance information.
It appears that no prior art monitoring platforms include the use of detailed system simulation modelling and/or statistical analysis in the process of monitoring the system's performance. The intended objective of these monitoring solutions is to detect sudden and significant change in performance or specific electrical faults. A core limitation of these monitoring units is that they are not capable of detecting smaller and more gradual drops in system performance. To do so would require sound comprehension of the operating behaviour of solar photovoltaic systems coupled with detailed manual analysis of data as presented by the monitoring unit.
Shortcomings of prior art monitoring systems include the requirement for detailed, manual analysis to detect the following:                Gradual declines in system performance, e.g. due to component degradation        Faults in the system other than that caused by the inverter or major faults        Fluctuating performance, e.g. due to out of range operating voltage as a result of poor system sizing        Temporary outages, e.g. due to grid instability causing the inverter to switch off for a short period of time        Changes to site specific conditions such as shading or soiling        